1. Field of the Invention
The present invention relates to a process that converts hydrocarbon or oil-based fuels to a variable mix of methane and hydrogen for solid oxide fuel cell (“SOFC”) applications.
2. Description of Related Art
High temperature solid oxide fuel cells (“SOFCs”) are energy conversion devices that directly convert chemical energy contained in the fuel feed to electrical energy. Being electrochemical devices, the conversion process in SOFCs is not limited by the Carnot heat cycle and can therefore achieve significantly higher efficiencies than conventional combustion engines or power stations.
Much of the momentum for SOFC development has been generated from environmental concerns because the high efficiency of SOFCs leads to lower CO2 emissions per kWh output of useful energy, while emissions of harmful chemicals such as NOx, SOx and un-burnt hydrocarbons are virtually zero. Applications targeted for SOFC include distributed and centralized power generation, vehicle propulsion, remote area power generation, and marine, military and aerospace applications.
Thermodynamic considerations suggest that SOFCs have the direct capability to electrochemically oxidize a wide variety of solid, liquid, and gaseous hydrocarbon fuels. However, most SOFC systems available to date still require these hydrocarbon fuels to be initially processed into more electrochemically active species such as carbon monoxide (CO) and hydrogen (H2). This processing requirement means that potentially any hydrogen and carbon containing materials could be used, provided it could be converted to CO and H2. For reasons of availability, transportability, and cost, natural gas and oil-based liquid petroleum fuel are seen as the most promising fuel options for SOFCs. The conversion process of these fuels to produce CO/H2 mixtures is known as “reforming.”
SOFCs have a number of characteristics which allow reforming to occur directly on the anode chamber: the catalyst which facilitates the electrochemical oxidation of CO/H2 mixtures is nickel based; nickel is the active metal in conventional steam reforming catalysts; the SOFC electrochemical reactions liberate heat and steam which are needed to support the endothermic reforming reaction; high conversions of hydrocarbons are further enhanced as CO and H2 product gases of the reforming reaction are continually consumed through the electrochemical oxidation reactions; and complete conversion of hydrocarbon based fuel is possible at temperatures as low as 700-750° C.
The process scheme allowing reforming to occur in the anode chamber is known as internal reforming. Conversely, external reforming refers to schemes in which the hydrocarbon fuel processing occurs in a separate unit located outside the solid oxide fuel cell module.
Internal reforming SOFC is the favored configuration as it offers significant advantages over its external counterpart. It provides superior net power output, lower supply rates of cathode air, greater amounts of quality waste heat, smaller footprint size and modularity of SOFC stacks. However, thermodynamic consideration suggests that internal reforming SOFC configuration may not be able to maintain SOFC stack temperature at part-load operating condition—a typical requirement for both grid-independent stationary and automotive applications.
One of the limitations of using SOFC technology in grid-independent power generation and automotive transportation is its inability to meet demanding variable load requirement, especially if an internal reforming SOFC stack is employed. While an internal reforming SOFC stack represents the most efficient fuel cell configuration, whereby the heat requirement for the steam reforming reaction can be met using the internal heat generation from the electrochemical reactions for electricity generation, maintenance of thermal balance under part-load condition (down to 20%) is extremely difficult. The internal heat generation by the SOFC stack during internal reforming is not sufficient to satisfy both the internal reforming heat requirement as a well as the SOFC stack's heat loss, which is generally fixed at a level corresponding to full-load operating conditions. One prior attempt to address this limitation is the inclusion of an external heat source that results in increased inefficiency of the SOFC stack, a more complex design, and eventually higher cost.
Another shortcoming of using an internal reforming SOFC operating on oil-based liquid fuels is the increased propensity for carbon and tar formation. Oil-based liquid fuels contain high molecular weight hydrocarbons which tend to crack at the high operating temperature of SOFC stack forming undesirable solid carbon. Over time, accumulated carbon blocks active sites for reactions on the anode surface, and often leads to an increased pressure drop that would alter the flow distribution of fuels to the different layers within the stack module.
Accordingly, prior to the development of the present invention, there has been no reforming SOFC process or reforming SOFC system that: provides for reforming at part-load operating conditions without the need for an external heat source; provides for an internal reforming SOFC stack to maintain thermal balance under part-load conditions; converts hydrocarbon fuels such as oil-based fuels, into a variable mixture of CH4, H2, and CO; exploits the superior performance of an internal reforming SOFC stack while achieving power generation under part-load conditions; and provides an internal reforming SOFC stack with a pre-processed feed stream having compositions that reduce formation of tar and carbon within the SOFC stack. Therefore, the art has sought a reforming SOFC process and a reforming SOFC system that: provides for reforming at part-load operating conditions without the need for an external heat source; provides for an internal reforming SOFC stack to maintain thermal balance under part-load conditions; converts hydrocarbon fuels such as oil-based fuels, into a variable mixture of CH4, H2, and CO; exploits the superior performance of an internal reforming SOFC stack while achieving power generation under part-load conditions; and provides an internal reforming SOFC stack that reduces formation of tar and carbon within the SOFC stack.